1. Field of the Invention
In the rubber industry use is made of great quantities of reclaimed rubber which is produced by degradation of rubber scrap, for example old car tires. Degradation of the rubber scrap is effected in conventional cases by the action of heat, mechanical processing and chemicals, in which process the textile and metal impurities included in the rubber are removed mechanically or chemically. Like raw rubber, reclaimed rubber is a plastic material which may be processed and vulcanized in the same manner as raw rubber.
2. Description of the Prior Art
There are six different conventional methods of reclaiming rubber scrap. The oldest method is the boiler or autoclave process in which the rubber scrap is heated in steam. In the autoclave process, use is made of high pressure steam at a temperature of approximately 200.degree. C. Another conventional process is the acid process in which all fabric accompanying the rubber scrap is dissolved by means of boiling in acids, whereafter the remainder is plasticized in steam and flattened down on a mill. A third method is the alkali method in which caustic soda solution is used instead of acid to dissolve the textile material. Besides, this method entails a removal of free sulphur. This method is not suitable in conjunction with chemical reclaiming agents and has, therefore, now lost importance with the increasing occurrence of synthetic rubber in the rubber scrap. A fourth method is the neutral process in which metal chlorides are used instead of alkali to dissolve the textile material. The neutral process is normally carried out in large boilers in steam heated water (temperature approx. 200.degree. C.), the textile material normally being dissolved by means of calcium chloride. A fifth method is the thermomechanical or thermochemical process in which the rubber scrap is processed at high temperature in a closed mixer, normally a propeller mixer or extruder, once the textile material has been mechanically removed.
In Chemical Abstracts, Vol. 84 (1976) col. 45783h is described a sixth method for reclaiming rubber scrap. This method may be described as direct reclaiming in the solid phase and comprises two operations, a first operation in which a mixture of natural rubber scrap in pulverulent form, process oil, phenyl hydrazine, ferrous chloride and methyl alcohol is agitated for 4 h, and a second operation during which the thus formed mixture is heated at 100.degree. C. for 60 min. This prior art sixth reclaiming method thus requires a relatively long treatment time, i.e. 5 h.
It may be said of all of these prior art reclaiming processes that the raw material (the rubber scrap) is first crushed and finely ground. In the autoclave process, the thermochemical process and the neutral process the material, after heat treatment, must also be subjected to an intensive mechanical processing in a mill and extruder, a treatment which is called "refining". This treatment consumes much energy and work, since the material is heated intensely during the processing in the refining mill and the extruder. A great amount of heat energy must, thus, be dissipated by water-cooling. Furthermore, the refinery mill has to be supervised, which requires great skill in adjusting the mill to achieve the best results.
In a further development of the conventional neutral process, the autoclaved rubber material has, after hot air drying, instead been exposed to cryomilling in a mill, for example a vibration mill or a rotary blade mill, in which the material is kept cooled down to below -60.degree. C. by the supply of some coolant, in particular liquid nitrogen or carbonic acid. Granted, this cryomilling achieves a considerable energy saving, but even this prior art variant of the neutral process entails a high energy consumption.
The disadvantages inherent in the first five conventional reclaiming processes are, thus, primarily high production costs in the form of wages and energy consumption. Another disadvantage inherent in these processes is that the rubber scrap, after the actual reclaiming, i.e. degradation of the double bonds in the rubber material, need be subjected to a special refining process in order to be usable as a raw material for new vulcanized rubber products.
Direct reclaiming in accordance with the above-mentioned sixth method (Chemical Abstracts, Vol. 84) entails, it is true, an improvement on the other prior art reclaiming methods, but still necessitates a relatively long treatment time and unnecessary working operations.